Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed abnormal combustion events, such as due to pre-ignition. The early abnormal combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Such abnormal combustion events can cause rapid engine degradation. Accordingly, strategies have been developed for early detection and mitigation of abnormal combustion events based on engine operating conditions.
One example approach is illustrated by Hashizume in U.S. Pat. No. 5,632,247. Therein, abnormal cylinder combustion due to pre-ignition and/or knock is detected by a knock sensor attached to the cylinder block. Specifically, based on an estimation of the knock sensor reading in two different timing windows, each with differing thresholds, pre-ignition events are determined and differentiated from knock.
However, the inventors herein have identified potential issues with such an approach. In one example, the approach requires substantial signal processing to differentiate abnormal combustion due to pre-ignition from abnormal combustion due to knock before an appropriate mitigating action can be performed. As such, this may add complexity to the detection and differentiation of combustion events. As another example, the approach uses distinct, non-overlapping windows. However, there may be regions in the knock window where pre-ignition may be better identified and vice versa. The sensitivity of the approach may also vary depending on the positioning of the sensor. Overall, system complexity and cost is increased without necessarily improving the performance of either knock or pre-ignition detection in the cylinder. As such, the reduced accuracy of engine pre-ignition determination and differentiation (from knock) may lead to rapid engine degradation. Additionally, distinguishing between knock and pre-ignition can be inaccurate and lead to incorrect actions being taken for each type of event, leading to degraded performance.
In one example, some of the above issues may be addressed by a method for detecting and addressing abnormal combustion. The method comprises: in response to a knock sensor output intensity occurring within a first window for a given cylinder combustion event, enriching the cylinder as a function of the output intensity. The method further comprises, in response to the knock sensor output intensity occurring within a second window for the combustion event being higher than a threshold, retarding spark ignition timing, the first window partially overlapping the second window. In this way, abnormal combustion due to knock and/or pre-ignition may be better addressed.
As an example, an engine system may include one or more knock sensors arranged in, at, or along an engine block or coupled to engine cylinders. Knock sensor output generated in one or more of a first and second crank angle timing window may be used to address abnormal combustion, such as those due to knock and/or pre-ignition. The first and second windows may be partially overlapping with the first window starting before the second window starts, the first window ending before the second window ends. The windows may be defined by specified crank angles as a function of engine operating conditions, such as engine speed and load. For a given cylinder combustion event, the first crank angle timing window may start before a cylinder spark ignition event (such as at 15 degrees ATC) and end in an expansion stroke of the cylinder (such as at 40 degrees ATC), while the second crank angle timing window may start after the cylinder spark ignition event and end in the expansion stroke, after the first window ends. Sensor output generated in the first and second windows may be processed (e.g., band pass filtered, rectified, and integrated) to determine respective output intensities. Based on the output intensity in the first window, a first set of abnormal combustion mitigating actions (e.g., pre-ignition mitigating actions) may be determined. For example, the cylinder may be enriched, with an amount of enrichment to be applied (degree of richness, number of enriched engine cycles, number of engine cylinders to be enriched, etc.) determined as a function of the output intensity. A look-up table stored in the controller's memory as a function of engine speed and knocking intensity may be used to determine the enrichment. In addition, as the number of enrichment cycles exceeds a threshold, an amount of engine load limiting may be applied. Further, spark timing may be adjusted (e.g., advanced) based on the enrichment applied to recover torque lost from operating richer than RBT. Based on the output intensity in the second window being higher than a threshold, a different set of abnormal combustion mitigating actions (e.g., knock mitigating actions) may be determined. For example, spark ignition timing may be retarded, an amount of spark retard applied increased as the output intensity exceeds the threshold in the second window. Thus, as the output intensity of the knock sensor increases in the first and/or second windows, a proportionally higher and more severe mitigating action may be performed.
The inventors herein have further recognized the synergistic relationship between abnormal combustion mitigating actions, such as those that address knock and those that address pre-ignition. Specifically, as a cylinder enrichment is increased (in proportion to increased knocking intensity), the resulting cylinder charge cooling reduces the likelihood of further abnormal combustion events in the cylinder (such as those due to knock) while also increasing tolerance of spark timing advance. As a result, as the cylinder enrichment determined based on the output in the first window increases (e.g., exceeds a threshold level), the amount of spark retard applied in response to the output in the second window may be decreased.
In this way, abnormal cylinder combustion may be addressed while reducing the complexity of knock sensor output processing. For example, abnormal combustion due to abnormal combustion events such as one or more of knock and pre-ignition may be mitigated without necessitating differentiation of the signals. In addition, the need for multiple knock sensors, multiple knock sensing windows, or multiple thresholds is reduced. By adjusting a severity of mitigating actions applied in response to an abnormal combustion event based on the output intensity of a knock sensor in defined windows, abnormal combustion due to each of knock and pre-ignition can be addressed without requiring knock and pre-ignition differentiation. By using partially overlapping windows, the accuracy of abnormal combustion detection can be improved. By increasing an enrichment and load limiting applied to an engine as the knock sensor output intensity in a first, earlier window increases, abnormal combustion induced further mega-knock events can be pre-empted. By adjusting an amount of spark timing retard applied based on the knock sensor intensity in a second, later window, partially overlapping the first window, knocking can be addressed. By using the same knock sensor to address different kinds of abnormal cylinder combustion events, component reduction benefits may be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.